Returns the transform X_PF giving the pose of the joint's "fixed" frame F in its parent body frame P.

Frame F is the joint frame that is fixed to the parent body; thus X_PF is not configuration dependent.

To clarify the sense of the returned transform X_PF, consider the location of a point Q somewhere in space. Let p_PQ be point Q measured and expressed in frame P and p_FQ be point Q measured and expressed in frame F. Then p_PQ is given by: